Optimization of low-background alpha spectrometers for analysis of thick samples

•Underground alpha spectrometry of thick sources.•Optimization of sensitivity of alpha spectrometers.•Reconstruction of the Po-210 diffusion profile in lead.

Results of alpha spectrometric measurements performed deep underground and above ground with and without active veto show that the underground measurement of thick samples is the most sensitive method due to significant reduction of the muon-induced background. In addition, the polonium diffusion requires for some samples an appropriate selection of an energy region in the registered spectrum. On the basis of computer simulations the best counting conditions are selected for a thick lead sample in order to optimize the detection limit.